Voice traffic gateway

ABSTRACT

A voice traffic gateway including: a voice traffic input, a voice traffic output, a first domain router, non-secure warning (NSW) tone application apparatus and a signalling filter. The input receives voice traffic including voice messages and signalling messages from a first traffic domain. The output delivers output voice messages and signalling messages to a second, different traffic domain. The first domain router routes voice messages to the NSW apparatus and routes signalling messages to the signalling filter. The NSW apparatus applies a NSW tone to a received voice message to form an output voice message. The signalling filter compares a feature of a received signalling message with a reference feature. The signalling filter will pass a signalling message if the signalling message feature is the same as the reference feature and will block a signalling message if the signalling message feature is not the same as the reference feature.

The invention relates to a voice traffic gateway.

It is known for communications networks to incorporate a number ofseparate traffic domains, often referred to as virtual private networks,connecting sub-sets of subscribers to a communications network. It isgenerally intended that voice call traffic flowing within a trafficdomain remains within that domain only, however situations can arise inwhich a subscriber in one domain wishes to call a subscriber in adifferent domain. In communications networks in which subscribers in thedifferent traffic domains have different privileges or security levelsplacing a call from one domain to another can give rise to a securitythreat, since the existence of such a call may allow a covertcommunications channel to be established between the two trafficdomains.

In addition, it may be desirable during a telephone call betweensubscribers in two different domains to be able to indicate to onesubscriber to the call that the other subscriber to the call is locatedin a domain having a different privilege or security level, and istherefore a non-trusted party. This is known to be done in the case ofanalogue voice calls by applying a non-secure warning tone to the voicecall received from the non-trusted party, so that an audible warning isheard over the call by the receiving party.

A first aspect of the invention provides a voice traffic gatewaycomprising:

a voice traffic input arranged to receive voice traffic from a firsttraffic domain of a communications network, the voice traffic comprisingvoice messages and signalling messages;

a voice traffic output arranged to deliver output voice messages andoutput signalling messages to a second, different traffic domain of thecommunications network;

a first domain router coupled to the voice traffic input and arranged toroute voice messages received from the first traffic domain to anon-secure warning tone application apparatus and to route signallingmessages received from the first traffic domain to a signalling filter,

wherein the non-secure warning tone application apparatus is arranged toapply a non-secure warning tone to each voice message that it receivesto form a respective output voice message,

and the signalling filter is arranged to compare a feature of eachsignalling message that it receives with a reference feature and isfurther arranged to pass a said signalling message as an outputsignalling message if said feature is the same as the reference featureand to block a said signalling message if said feature is not the sameas the reference feature.

The voice traffic gateway may enable voice calls to be made betweencallers in two different traffic domains of a communications network.The non-secure warning tone application apparatus may enable the voicetraffic gateway to apply a warning tone to a voice call to alert arecipient to the fact that the call has originated from a differenttraffic domain. The signalling filter may enable the voice securitygateway to prevent a covert message being transmitted with a signallingmessage and may prevent a cover communications channel being establishedbetween the two different traffic domains.

In an embodiment, each voice message and signalling message receivedfrom the first traffic domain comprises a first domain identificationlabel, and the voice traffic input and the voice traffic output comprisea shared communications port, and wherein the voice traffic gatewayfurther comprises:

a first domain separator arranged to receive the voice traffic and toread the domain identification label of each voice message and eachsignalling message, and remove the first domain identification labelfrom each voice message and each signalling message having anidentification label of the first traffic domain; and

a second domain separator arranged to apply a second domainidentification label to each voice message and each signalling messageto be output to the second domain; and

routing apparatus arranged to route voice traffic received from thefirst domain from the communications port to the first domain separatorand to route voice traffic to be output to the second domain from thesecond domain separator to the communications port.

The voice traffic gateway may be used with a communications network inwhich voice traffic belonging to the two different traffic domains istransmitted across a shared network, the traffic of each domain beingidentified by its respective domain identification label.

In an embodiment, the voice traffic gateway is arranged to receive voicetraffic from each of a plurality of different traffic domains and tooutput voice traffic to each of said traffic domains and the voicetraffic gateway comprises:

a said plurality of domain separators each arranged to receiverespective voice traffic and to read the domain identification label ofeach voice message and each signalling message of said voice traffic,and to remove the respective domain identification label from each voicemessage and each signalling message having an identification label ofits respective domain and to apply said respective domain identificationlabel to each voice message and each signalling message to be output toits respective domain;

a said plurality of non-secure warning tone application apparatus andsignalling filters;

a said plurality of domain routers, each coupled to a respective domainseparator and to at least one other domain router through a respectivesaid non-secure warning tone application apparatus and a respective saidsignalling filter,

and wherein each domain router is arranged to route voice trafficreceived from its respective domain separator to a respective saidnon-secure warning tone application apparatus or a respective saidsignalling filter and each domain router is further arranged to routevoice traffic received from a respective said non-secure warning toneapplication apparatus or a respective said signalling filter to itsrespective said domain separator.

The voice traffic gateway may be used with a communications network inwhich voice traffic belonging to each of a plurality of differenttraffic domains is transmitted across a shared network, the traffic ofeach domain being identified by its respective domain identificationlabel. The voice traffic gateway may be used to enable communicationbetween any pre-selected pair of the plurality of different trafficdomains within the communications network.

In an embodiment, the routing apparatus is further arranged to decryptvoice traffic received from the communications port prior to routingsaid traffic to the respective domain separator and is further arrangedto encrypt traffic to be output prior to routing said traffic to thecommunications port. The voice traffic gateway may therefore be used toenable encrypted communication between two different traffic domainswithin a communications network

In an embodiment, said feature of each signalling message comprises atleast one of a source address, a destination address, signalling messageformat, the content of a signalling message field. The signalling filtermay therefore be arranged only to pass signalling messages whichoriginate from an acceptable source, which are addressed to anacceptable destination, which have an acceptable message format or whichhave acceptable content in one or more pre-selected message fields.

In an embodiment, the signalling messages have a bit rate and thesignalling filter is further arranged to control the bit rate to be notgreater than a pre-selected threshold bit rate. This may enable thesignalling filter to throttle the bit rate of a signalling message to aminimum bit rate required to successfully transmit signalling messagesand may thereby disrupt transmission of covert messages with signallingmessages.

In an embodiment, the signalling messages comprise signalling packetsand the signalling filter is arranged to: convert received signallingpackets into a first serial bit stream; convert the first serial bitstream into a second serial bit stream having a pre-selected, lower bitrate; and convert the second serial bit stream into output signallingpackets. The bit rate of a signalling message may thereby be reduced toa pre-selected lower bit rate, throttling the bit rate of the signallingmessage.

In an embodiment, each traffic domain has a trust level and thenon-secure warning tone apparatus is operable to apply the non-securewarning tone only to messages being sent from a traffic domain having afirst trust level to a traffic domain having a second, higher trustlevel. Only the more trusted party to a call may hear the non-securewarning tone, alerting them that the other caller is less trusted,without the less trusted caller being aware that they are talking to amore trusted party.

In an embodiment, each voice message comprises a voice signal comprisinga sequence of voice samples and the non-secure warning tone applicationapparatus comprises:

a port arranged to receive a said voice signal;

a non-secure warning tone signal source arranged to generate anon-secure warning tone signal comprising a plurality of tone samples;and

signal modification apparatus arranged to selectively include tonesamples in the sequence of voice samples.

The non-secure warning tone application apparatus may apply a non-securewarning tone to a packetized voice signal while minimising disruption ofthe voice signal. The apparatus may modify a voice signal by includingtone samples which may corrupt a data signal comprising data sampleswhich is being transmitted across a voice signal path through thenon-secure warning tone application apparatus, and may prevent a datasignal from being covertly transmitted across the voice signal path.

In an embodiment, the signal modification apparatus is arranged toselect a plurality of the voice samples and replace each selected voicesample with a respective tone sample. The tone samples may be providedwith a constant amplitude which may ensure that they are always audibleand easily recognised. Replacement of voice samples with tone samplesmay enable a caller to adjust the voice call volume relative to thenon-secure warning tone volume. Replacement of voice samples with tonesamples may provide a resulting voice call having a non-secure warningtone which is easy and pleasant to listen to.

In an embodiment, the signal modification apparatus is arranged toselect a plurality of the voice samples and to combine each selectedvoice sample with a respective tone sample to form a combined sample.This may enable the relative and absolute levels of the tone samples andthe voice samples to be varied while clamping the minimum and maximumnon-secure warning tone amplitude on the voice call between pre-selectedlimits.

In an embodiment, each combined sample comprises a weighted average ofthe respective voice sample and tone sample.

In an embodiment, the signal modification apparatus is arranged toselect alternate voice samples. A non-secure warning tone may thereby beapplied to the voice signal which may be heard without obscuring thevoice call and may maximise the intelligibility of a voice call having anon-secure warning tone on it.

In an embodiment, the signal modification apparatus is arranged toselect one in up to every sixteen voice samples. In an embodiment, thesignal modification apparatus is arranged to select one in up to everyfour voice samples. This may reduce the amount of signal processingrequired without affecting the intelligibility of the voice call.

In an embodiment, the non-secure warning tone application apparatus isarranged to periodically modify the voice signal. The non-secure warningtone application apparatus may thereby apply a non-secure warning toneduring only selected parts of a voice call. A periodically repeatingnon-secure warning tone may thus be applied to a packetized voicesignal. A non-secure warning tone may therefore be applied which may notdistract a receiving party from the content of a voice call.

In an embodiment, the non-secure warning tone signal source is arrangedto generate a tone signal comprising periodically occurring tone periodseach comprising a plurality of tone samples and the signal modificationapparatus is arranged to selectively include tone samples in thesequence of voice samples during the tone periods. A periodicmodification of the voice signal is therefore achieved modifying thevoice signal with a periodic tone signal, which only comprises tonesamples within periodically occurring tone periods.

In an embodiment, the voice messages and the signalling messages have avoice over internet protocol signal format and comprise voice messagepackets and signalling message packets.

In an embodiment, the sequence of voice samples comprises a sequence ofvoice message packets and the apparatus further comprises voice signalvalidation apparatus arranged to compare a structural feature of a voicemessage packet of the voice signal with a reference structural featureand arranged to determine whether said structural feature is the same assaid reference feature. If said structural feature is the same as saidreference feature the voice signal validation apparatus may validate thevoice message packet as comprising an acceptable voice message packet.If said structural feature is not the same as said reference feature thevoice signal validation apparatus may reject the voice message packet ascomprising an acceptable voice message packet. This may preventunacceptable voice message packets or data packets from beingtransmitted by the non-secure warning tone application apparatus.

In an embodiment, said structural feature comprises at least one ofpacket length, packet header and voice header.

In an embodiment, each voice message packet has a packet length ofbetween 10 ms and 50 ms. In an embodiment, the non-secure warning tonesignal has a lower signal level than the voice signal. The non-securewarning tone may therefore be heard in the background of a voice call.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a voice traffic gatewayaccording to a first embodiment of the invention;

FIG. 2 is a schematic representation of a voice traffic gatewayaccording to a second embodiment of the invention;

FIG. 3 is a schematic representation of a voice traffic gatewayaccording to a third embodiment of the invention;

FIG. 4 is a schematic illustration of a non-secure warning toneapplication apparatus of a voice traffic gateway according to a fourthembodiment of the invention;

FIG. 5 is a graphical representation of audio patterns of (a) a voicecall and (b) a non-secure warning tone signal to be merged with thevoice call; and

FIG. 6 is a schematic illustration of a non-secure warning toneapplication apparatus of a voice traffic gateway according to a fifthembodiment of the invention.

Referring to FIG. 1, a first embodiment of the invention provides avoice traffic gateway 10 comprising a voice traffic input 12, a voicetraffic output 14, a first domain router 16, non-secure warning toneapplication apparatus 18 and a signalling filter 20.

The voice traffic input 12 is arranged to receive voice traffic from afirst traffic domain of a communications network. The voice trafficcomprises voice messages and signalling messages. The voice trafficoutput 14 is arranged to deliver output voice messages and outputsignalling messages to a second, different traffic domain of thecommunications network.

The first domain router 16 is coupled to the voice traffic input and isarranged to route voice messages received from the first traffic domainto the non-secure warning tone application apparatus 18. The firstdomain router 16 is further arranged to route signalling messagesreceived from the first traffic domain to the signalling filter 20.

The non-secure warning tone application apparatus 18 is arranged toapply a non-secure warning tone to a received voice message to form anoutput voice message.

The signalling filter 20 is arranged to compare a feature of a receivedsignalling message with a reference feature. The signalling filter 20will pass a signalling message as an output signalling message if thesignalling message feature is the same as the reference feature. Thesignalling filter 20 will block a signalling message if the signallingmessage feature is not the same as the reference feature.

A second embodiment of the invention provides a voice traffic gateway 30as shown in FIG. 2. The voice traffic gateway 30 of this embodiment issimilar to the voice traffic gateway 10 of FIG. 1 with the followingmodifications. The same reference numbers are retained for correspondingfeatures.

In this embodiment, the voice traffic input and the voice traffic outputare provided as a shared communications port 32, arranged to beconnected to a transmission network shared by both the first and secondtraffic domains.

The voice traffic gateway 30 comprises a first domain separator 36 and asecond domain separator 38. The first domain separator is arranged toreceive voice traffic and to read the domain identification label ofeach voice message and each signalling message received. The firstdomain separator is arranged to remove the domain identification labelfrom each voice message and each signalling message having anidentification label of the first traffic domain.

The second domain separator 38 is arranged to apply a second domainidentification label to each voice message and each signalling messageto be output to the second domain through the communications port 32.

The voice traffic gateway 30 further comprises routing apparatus 34arranged to route voice traffic of the first domain received from thecommunications port to the first domain separator 36. The routingapparatus 34 is further arranged to route voice traffic to be output tothe second domain from the second domain separator 38 to thecommunications port 32.

FIG. 3 shows a voice traffic gateway 40 according to a third embodimentof the invention. The voice traffic gateway 40 of this embodiment issimilar to the voice traffic gateway 30 of FIG. 2 with the followingmodifications. The same reference numbers are retained for correspondingfeatures.

In this embodiment, the voice traffic has a voice over internetprotocol, VoIP, signal format and comprises voice message packets andsignalling message packets.

The routing apparatus comprises encryption and routing apparatus 56. Theencryption and routing apparatus 56 is arranged to decrypt voice trafficreceived from the communications 32 port and to route the decryptedtraffic to the respective domain separator 36, 38, 42. The encryptionand routing apparatus 56 is additionally arranged to encrypt traffic tobe output prior to routing the encrypted output traffic to thecommunications port 32.

The voice traffic gateway 40 is arranged to receive voice traffic fromeach of three different traffic domains and to output voice traffic toeach of the three traffic domains. The voice traffic gateway 40comprises three domain separators 36, 38, 42, three domain routers 16,44, 46 and three sets of non-secure warning tone application apparatus18, 48, 52 and signalling filters 20, 50, 54.

Each domain separator 36, 38, 42 is arranged to receive voice trafficand to read the domain identification label of each voice message andeach signalling message of the voice traffic. Each domain separator 36,38, 42 is arranged to remove the respective domain identification labelfrom each voice message and each signalling message having anidentification label of its respective domain. Each domain separator 36,38, 42 is additionally arranged to apply its respective domainidentification label to each voice message and each signalling messageto be output to its respective domain.

Each domain router 16, 44, 46 is coupled to a respective domainseparator 36, 38, 42. Each domain router 16, 44, 46 is also coupled toat least one other domain router 16, 44, 46 through a non-secure warningtone application apparatus 20, 50, 54 and said signalling filter 18, 48,52.

The first domain separator 36 and the first domain router 16 belong to afirst traffic domain. The second domain separator 38 and the seconddomain router 44 belong to a second traffic domain. The third domainseparator 42 and the third domain router 46 belong to a third trafficdomain.

The first domain router 16 is connected to the second domain router 44through a first non-secure warning tone application apparatus 20 and afirst signalling filter 18. The second domain router 44 is connected tothe third domain router 46 through a second non-secure warning toneapplication apparatus 50 and a second signalling filter 48. The firstdomain router 16 is also connected to the third domain router 46 througha third non-secure warning tone application apparatus 54 and a thirdsignalling filter 52.

Each domain router 16, 44, 46 is arranged to read a destination addressof each voice message and each signalling message received from itsrespective domain separator 36, 38, 42. The destination address isindicative of the destination domain. Each domain router 16, 44, 46 isarranged to route the voice messages and signalling messages to therespective non-secure warning tone application apparatus 20, 50, 54 orsignalling filter 18, 48, 52 connected to the domain separator 36, 38,42 of the destination domain.

Each domain router 16, 44, 46 is arranged to default route voicemessages and signalling messages received from a respective non-securewarning tone application apparatus 20, 50, 54 or signalling filter 18,48, 52 to the encryption and routing apparatus 56, through itsrespective domain separator 36, 38, 42.

Each non-secure warning tone application apparatus 20, 50, 54 and eachsignalling filter 18, 48, 52 is operable for bi-directionalcommunication. Voice messages can therefore be transmitted from thefirst domain router 16 to the second domain router 44 through the firstnon-secure warning tone application apparatus 20, and vice versa. Voicemessages can be transmitted from the first domain router 16 to the thirddomain router 46 through the third non-secure warning tone applicationapparatus 54, and vice versa. Voice messages can be transmitted from thesecond domain router 44 to the third domain router 46 through the secondnon-secure warning tone application apparatus 50, and vice versa.

Similarly, signalling messages can therefore be transmitted from thefirst domain router 16 to the second domain router 44 through the firstsignalling filter 18, and vice versa. Signalling messages can betransmitted from the first domain router 16 to the third domain router46 through the third signalling filter 52, and vice versa. Signallingmessages can be transmitted from the second domain router 44 to thethird domain router 46 through the second signalling filter 48, and viceversa.

Voice traffic may therefore be transmitted between any pair of the threetraffic domains.

A fourth embodiment of the invention provides a voice traffic gatewaycomprising non-secure warning tone application apparatus 60 as shown inFIG. 4. The non-secure warning tone application apparatus 60 comprises aport 64, a non-secure warning tone signal source 66 and signalmodification apparatus 68.

The port 64 is arranged to receive a voice message 62 comprising a voicesignal comprising a sequence of voice samples.

The non-secure warning tone signal source 66 is arranged to generate anon-secure warning tone signal 70 comprising a plurality of tonesamples. The signal modification apparatus 68 is arranged to modify thevoice signal 62 by selectively including tone samples of the tone signal70 in the sequence of voice samples.

A fifth embodiment of the invention provides a voice traffic gatewaycomprising non-secure warning tone application apparatus which issimilar to the apparatus 60 shown in FIG. 4, with the followingmodifications. This embodiment will be described with reference to FIG.4.

In this embodiment, the voice traffic has a voice over internetprotocol, VoIP, signal format and comprises voice message packets andsignalling message packets. The signal modification apparatus 68 isarranged to select some of the voice samples of the voice signal 62 andto replace each selected voice sample with a respective tone sample.

An sixth embodiment of the invention provides a voice traffic gatewaycomprising non-secure warning tone application apparatus which issimilar to the apparatus 60 shown in FIG. 4, with the followingmodifications. This embodiment will be described with reference to FIG.4.

In this embodiment, the voice traffic has a voice over internetprotocol, VoIP, signal format and comprises voice message packets andsignalling message packets. The signal modification apparatus 68 isarranged to select some of the voice samples of the voice signal 62 andto combine each selected voice sample with a respective tone sample toform a combined sample. In this example, the signal modificationapparatus 68 is arranged to combine the voice and tone samples by takinga weighted average of each pair of voice sample and tone samples. Theresulting voice signal therefore comprises a sequence of voice samplesand combined samples.

A seventh embodiment of the invention provides a voice traffic gatewaycomprising non-secure warning tone application apparatus which issimilar to the apparatus 40 shown in FIG. 4, with the followingmodifications. This embodiment will be described with reference to FIG.4.

In this embodiment, the voice traffic has a voice over internetprotocol, VoIP, signal format and comprises voice message packets andsignalling message packets. The signal modification apparatus 68 isarranged to modify the voice signal 62 periodically, so that anon-secure warning tone is only applied to selected periods of a voicecall, referred to as tone periods.

The method of this embodiment comprises modifying the voice signal onlyduring tone periods. The tone periods are periodically occurring so thata periodic non-secure warning tone is applied to the voice signal.Applying a periodic non-secure warning tone may enable a receiving partyto be made aware that they are holding a call with a non-trusted partywithout distracting the receiving party from the content of the call.This may ensure that the non-trusted party does not receive anyindication (by, for example, the receiving party missing part of what isbeing said by the non-trusted party and requesting them to repeat it)that a non-secure warning tone is being heard by the receiving party.The non-trusted party may therefore be kept unaware of their non-trustedstatus.

Referring to FIG. 5, which illustrates the audio patterns of the tonesignal 70 and the voice signal 62, the tone signal 70 provides a tonesamples during tone periods 72 a, 72 b, which repeat at a periodicity P.In this example, the non-secure warning tone signal source 66 isarranged to generate first tone samples arranged to provide a tonesignal 70 at a first frequency tone, for example “beep”, in the firsttone periods 72 a and to generate second tone samples arranged toprovide a second frequency tone, for example “bop”, in the second toneperiods 72 b. This results in a repeating “beep” “bop” non-securewarning tone being applied to the voice signal 62.

An eighth embodiment of the invention provides a voice traffic gatewaycomprising non-secure warning tone application apparatus which issimilar to the apparatus 60 of the previous embodiment, with thefollowing modifications. This embodiment will also be described withreference to FIG. 4.

In this embodiment, the voice signal has a voice over internet protocol(VoIP) signal format and comprises voice message packets and signallingmessage packets. The signal modification apparatus 68 is arranged toselect alternate ones of the voice samples within each tone period andto replace each selected voice sample with a respective tone sample. Anon-secure warning tone may thereby be applied to the voice signal whichmay be heard without obscuring the voice call and may maximise theintelligibility of a voice call having a non-secure warning tone on it.

A ninth embodiment of the invention provides a voice traffic gatewaycomprising non-secure warning tone application apparatus which issimilar to the apparatus 60 of the previous embodiment, with thefollowing modifications. This embodiment will also be described withreference to FIG. 4.

In this embodiment, the voice signal has a voice over internet protocol(VoIP) signal format. The signal modification apparatus 68 is arrangedto select every fourth voice sample within each tone period and toreplace each selected voice sample with a respective tone sample. Thismay reduce the amount of signal processing required by the signalmodification apparatus 68 without affecting the intelligibility of thevoice call.

The signal merging apparatus 68 may alternatively be arranged to selectup to every sixteenth voice sample.

The non-secure warning tone signal source 66 is arranged to generate anon-secure warning tone signal 70 having a lower signal level than thevoice signal 62 so that the voice call can be clearly heard over thenon-secure warning tone during tone periods. This is advantageous inensuring that the voice call is not corrupted or obscured during toneperiods.

Referring to FIG. 6, a tenth embodiment of the invention provides avoice traffic gateway comprising non-secure warning tone applicationapparatus 80 which is similar to the non-secure warning tone applicationapparatus 60 shown in FIG. 4, with the following modifications. The samereference numbers are retained for corresponding features.

In this embodiment, the voice signal has a voice over internet protocol(VoIP) signal format and comprises voice message packets and signallingmessage packets. The sequence of voice samples is provided as a sequenceof voice message packets. Each voice message packet has a packet lengthof 20 ms and comprises 160 voice samples each having a sample length of0.125 ms. The apparatus 80 further comprises voice signal validationapparatus 82 arranged to compare a structural feature of a voice messagepacket of the voice signal 62 with a reference structural feature. Thestructural feature is selected from one of packet length, packet headerand voice header. A voice message packet has a well known structure andwill have a pre-selected length, packet header and voice header, eachwhich may be stored as a reference structural feature. It will beappreciated that one or more of the structural features may be comparedwith the corresponding reference structural feature during thevalidation step. Other structural features may alternatively be used.

The validation apparatus 82 is further arranged to determine whether theselected structural feature is the same as the reference feature. If thestructural feature is the same as the reference feature the voice signalvalidation apparatus 82 is arranged to validate the voice packet ascomprising an acceptable voice packet. If the structural feature is notthe same as the reference feature, the voice signal validation apparatus82 is arranged to reject the voice packet as comprising an acceptablevoice packet. The validation apparatus 82 is further arranged to discardany voice packet which is rejected as being acceptable. In this wayunacceptable voice packets or data packets may be prevented from beingtransmitted by the non-secure warning tone application apparatus 80.

The invention claimed is:
 1. A voice traffic gateway comprising: a voicetraffic input arranged to receive voice traffic from a first trafficdomain of a communications network, the voice traffic including voicemessages and signalling messages; a voice traffic output arranged todeliver output voice messages and output signalling messages to asecond, different traffic domain of the communications network; a firstdomain router coupled to the voice traffic input and configured to routevoice messages received from the first traffic domain to a non-securewarning tone application apparatus and to route signalling messagesreceived from the first traffic domain to a signalling filter; whereinthe non-secure warning tone application apparatus is configured to applya non-secure warning tone to each voice message that it receives to forma respective output voice message; and the signalling filter isconfigured to compare a feature of each signalling message that itreceives with a reference feature, to pass said signalling message as anoutput signalling message if said feature is the same as the referencefeature, and to block said signalling message if said feature is not thesame as the reference feature.
 2. A voice traffic gateway as claimed inclaim 1, configured to receive voice messages and signalling messagesfrom the first traffic domain which include a first domainidentification label, wherein the voice traffic input and the voicetraffic output are configured to use a shared communications port, thevoice traffic gateway comprising: a first domain separator configured toreceive the voice traffic and to read a domain identification label ofeach voice message and each signalling message, and to remove the firstdomain identification label from each voice message and each signallingmessage having a domain identification label of the first trafficdomain; a second domain separator configured to apply a second domainidentification label to each voice message and each signalling messageto be output to the second domain; and a routing apparatus configured toroute voice traffic received from the first traffic domain from thecommunications port to the first domain separator, and to route voicetraffic to be output to the second domain from the second domainseparator to the communications port.
 3. A voice traffic gateway asclaimed in claim 2, wherein the voice traffic gateway is arranged toreceive voice traffic from each of a plurality of different trafficdomains and to output voice traffic to each of said traffic domains, thevoice traffic gateway comprising: a plurality of domain separators eachconfigured to receive respective voice traffic and to read a domainidentification label of each voice message and each signalling messageof said voice traffic, and to remove the respective domainidentification label from each voice message and signalling messagehaving an identification label of its respective domain and to applysaid respective domain identification label to each voice message andeach signalling message to be output to its respective domain; aplurality of non-secure warning tone application apparatus andsignalling filters; and a plurality of domain routers, each coupled to arespective domain separator and to at least one other domain routerthrough a respective non-secure warning tone application apparatus and arespective signalling filter, wherein each domain router is configuredto route voice traffic received from its respective domain separator toa respective non-secure warning tone application apparatus or arespective signalling filter, and each domain router is configured toroute voice traffic received from a respective non-secure warning toneapplication apparatus or a respective signalling filter to itsrespective domain separator.
 4. A voice traffic gateway as claimed inclaim 2, wherein the routing apparatus is configured to decrypt voicetraffic received from the communications port prior to routing saidvoice traffic to a respective domain separator, and is to encrypt voicetraffic to be output prior to routing said voicetraffic to thecommunications port.
 5. A voice traffic gateway as claimed in claim 1,configured to receive signalling messages having at least one of asource address, a destination address, a signalling message format, andcontent of a signalling message field.
 6. A voice traffic gateway asclaimed in claim 1, configured to receive signalling messages having abit rate, wherein the signalling filter is configured to control the bitrate to be not greater than a pre-selected threshold bit rate.
 7. Avoice traffic gateway as claimed in claim 6, configured to receivesignalling messages having signalling packets, wherein the signallingfilter is configured to: convert received signalling packets into afirst serial bit stream; convert the first serial bit stream into asecond serial bit stream having a pre-selected, lower bit rate; andconvert the second serial bit stream into output signalling packets. 8.A voice traffic gateway as claimed in claim 1, wherein each trafficdomain has a trust level and the non-secure warning tone apparatus isconfigured to apply the non-secure warning tone only to messages beingsent from a traffic domain having a first trust level to a trafficdomain having a second, higher trust level.
 9. A voice traffic gatewayas claimed in claim 1, configured to receive voice messages whichinclude a voice signal comprising a sequence of voice samples, whereinthe non-secure warning tone application apparatus comprises: a portarranged to receive a said voice signal; a non-secure warning tonesignal source arranged to generate a non-secure warning tone signalhaving a plurality of tone samples; and a signal modification apparatusarranged to selectively include tone samples in the sequence of voicesamples.
 10. A voice traffic gateway as claimed in claim 9, wherein thesignal modification apparatus is configured to select a plurality of thevoice samples, and to replace each selected voice sample with arespective tone sample.
 11. A voice traffic gateway as claimed in claim9, wherein the signal modification apparatus is configured to select aplurality of the voice samples, and to combine each selected voicesample with a respective tone sample to form a combined sample.
 12. Avoice traffic gateway as claimed in claim 11, wherein each combinedsample will comprise: a weighted average of the respective voice sampleand tone sample.
 13. A voice traffic gateway as claimed in claim 10,wherein the signal modification apparatus is configured to selectalternate voice samples.
 14. A voice traffic gateway as claimed in claim10, wherein the signal modification apparatus is configured to selectone in up to every sixteen voice samples.
 15. A voice traffic gateway asclaimed in claim 9, wherein the sequence of voice samples comprises: asequence of voice message packets, and the signal modification apparatuscomprises: a voice signal validation apparatus configured to compare astructural feature of a voice message packet of the voice signal with areference structural feature, to determine whether said structuralfeature is the same as said reference feature, and if said structuralfeature is the same as said reference feature to validate the voicemessage packet as including an acceptable voice message packet.
 16. Avoice traffic gateway as claimed in claim 15, wherein said structuralfeature comprises: at least one of packet length, packet header andvoice header.